1. Field of the Invention
The present invention relates to a wavelength tunable laser suitable for multi-wavelength communication systems and so forth and a method of controlling the same.
2. Description of the Related Art
Along with dramatic increase in demands for communication in recent years, development of multi-wavelength communication systems (wavelength-division multiplexing (WDM) systems), which realize high-capacity transmission by a single optical fiber by way of multiplexing plural signal beams of different wavelengths, shows progress. For such multi-wavelength communication systems, a wavelength tunable laser capable of selecting a desired wavelength from a wide range of wavelengths is strongly expected in building the systems.
As a structure for realizing the wavelength tunable laser capable of selecting the desired wavelength from a wide range of wave lengths, as shown in FIG. 7, there is one having a resonator provided with two reflectors 106, 107, in which there are arranged a semiconductor optical amplifier (SOA) 101 having gain for a wide range of wavelengths, a phase shifter 102, a wavelength tunable filter 103 capable of selecting the desired wavelength from a wide range of wavelengths, and an etalon 105. An etalon is an optical filter having a periodical transmissive wavelength. In addition, outside the resonator, there are provided a beam splitter 109 for splitting part of a laser beam passing through the reflector 107, an optical detector 110 for detecting an optical intensity of the laser beam split by the beam splitter 109, and a phase control portion 111 for controlling phase condition, in other words the longitudinal-mode position, based on detection results by the optical detector 110.
In the wavelength tunable laser of such a structure, in order to control an oscillation wavelength, two types of controls described below are required. A first control is a coarse control to adjust the wavelength selected by the wavelength tunable filter 103 to a desired transmissive wavelength of the etalon 105. A second control is a fine control to adjust the oscillation wavelength to a desired wavelength (for example, the ITU grid.) by adjusting a longitudinal-mode position of the resonator. For the second control, there is provided inside the resonator the phase shifter 102 for controlling resonator length (phase), allowing the control of the longitudinal-mode position of the resonator.
For controlling phase, there are such techniques that integrate an optical waveguide for phase control with the SOA and change the resonator length by making use of the change in the refractive index of waveguide caused by current injection by disposing a semiconductor, as described in Patent document 1, or that change position of a mirror composing the resonator.
When controlling such a phase, conventional wavelength tunable lasers detect an intensity of a laser beam and perform feedback so that the intensity is maximized, as described in Patent document 2, or perform feedback so that a voltage applied to an active layer of the SOA is minimized, as described in Patent document 3. These phase controls are making use of such characteristics that the intensity of the laser beam is maximized and the voltage of the active layer of the SOA is minimized when the longitudinal-mode position of the resonator coincides with the transmissive peak wavelength of the etalon channel selected by the wavelength tunable filter.
However, such controls as maximizing the intensity of the laser beam or performing feedback to minimize the voltage of the SOA active layer have problems as described below.
First, because of the influence of asymmetric gain saturation in the SOA, the phase in which the laser beam intensity is maximized and the voltage of the SOA active layer is minimized deviates from a point enabling the most stabilized control. FIG. 8 is a view showing a change in optical output (laser beam intensity) when phase is changed. As shown in FIG. 8, the change in optical output with regard to phase (longitudinal-mode position) is periodical. Each range in which optical output is continuously changing corresponds to a range in which the oscillation of a longitudinal mode is performed, and the oscillation shifts to the other longitudinal mode at a point of a discontinuously changing phase. The change in optical output in the range in which oscillation is performed in a single longitudinal mode is asymmetric to the phase where optical output is maximized, and when the phase deviates from the phase to maximize the optical output toward the short wave side, the oscillation is immediately performed using the other longitudinal mode. Such a phenomenon occurs under the influence of the asymmetric gain saturation in the SOA, in which the gain in the oscillation wavelength on the short wave side falls and the same on a long wave side increases to thereby facilitate oscillation in a mode of the long wave side. This is unavoidable at this moment.
Therefore, in the technique which performs feedback to maximize optical output, the tolerance of phase position on the short wave side is smaller , so that a mode jump tends to occur. As a result, stable oscillation becomes difficult to thereby worsen noise characteristics of the laser. Such a phenomenon similarly occurs when monitoring the voltage of the active layer. Besides, a laser beam intensity change caused by other factor, for example deviation of the filter wavelength of the wavelength tunable filter, may affect sometimes.
Secondly, when the above-mentioned control is performed, it is required to control by a dithering with regard to the phase so that the change in laser beam intensity (optical output) or the slope of voltage change in the active layer comes to 0 (zero). However, the dithering with regard to the phase leads to a periodical change of the longitudinal position, whereby the oscillation wavelength varies.
Prior Arts are disclosed in Patent document 1 (Japanese Patent Application Laid-Open No. Hei 7-335965), Patent document 2 (U.S. Patent Application Publication No. 2003/0142700A1), Patent document 3 (U.S. Pat. Ser. No. 4,622,672), Patent document 4 (Japanese Patent Application Laid-Open No. 2001-85774), Patent document 5 (Japanese Patent Application Laid-Open No. 2001-251013), Patent document 6 (Japanese Patent Application Laid-Open No. 2002-185074), and Non-Patent document 1 (Sharon et al. in Journal of the Optical Society of America A, vol. 14, No. 11, pp. 2985–2993 (1997)).